The author has obviously done a lot of homework. He missed mentioning the conclusion of U.S. Grant's memoirs where he concludes after long and careful consideration that the cause of the war was in fact slavery. The quotes the author cherry picked from Lincoln's did not adress the authors point directly, Grant however did.
Slavery was obviously a large contributing factor to the Civil War, but it was hardly the only factor. The north and south were divided on a number of issues and the conflict might have occurred even if slaves never were part of the picture.
A long and careful consideration yes but keep in mind the institution of slavery was not the central issue but rather the tariffs imposed upon slave states. The north was not against slavery because of strictly humanitarian reasons. When war was declared the north had more slaves than did the South.
There is no historical record of any kind that shows that the North went to war to rid the US of slavery. The North went to war to preserve the Union. The South, OTOH, was content to withdraw from the Union and go its own way. Ft. Sumpter was probably the biggest boner of the Confederacy. Had it not occurred..., well I can't really say that war wouldn't have happened since Lincoln had a war plan drawn up.